With transmissions of the planetary-gear type it is known to establish different speed ratios between an engine and a load, such as the traction wheels of a vehicle, with the aid of hydraulic coupling elements such as clutches and brakes (also referred to as drive-establishing means) which are activable and deactivable by means of individual valve-operating switches responding to respective control signals from a selector circuit. In a system with three forward speed ratios or "gears", e.g. as described in commonly owned U.S. Pat. No. 3,580,109, two brakes and a clutch are used as the coupling elements; a fourth speed ratio can be realized with the addition of a third brake, e.g. as described in commonly owned U.S. Pat. No. 3,820,418, and so on. In each instance, only one of these coupling elements is activated at any time to establish the selected speed ratio; thus, a changeover from one speed ratio to another requires the deactivation of one coupling element simultaneously with or slightly before activation of another one.
For smooth shifting it is desirable that certain operating conditions be taken into account, particularly the sign of the reaction torque developed by the load. With positive reaction torques, as during acceleration or uphill driving of an automotive vehicle, upshifting-- i.e. the transition from a lower to a higher speed ratio-- should occur without significant hiatus in order to prevent any appreciable loss of momentum at a time when neither the lower-ranking nor the higher-ranking coupling stage is active. If, however, a downshift is to be carried out under load (i.e. in the presence of a positive reaction torque), such loss of momentum affords a smoother transition from a higher to a lower speed ratio; a short pause between the deactivation of a higher-ranking coupling element and the activation of a lower-ranking one is therefore not only permissible but also desirable under these circumstances. In the presence of negative reaction torques, i.e. with the load pulling (as during downhill driving or deceleration), the opposite is the case; a hiatus between deactivation of one coupling element and activation of another is then advantageous during upshifting but not during downshifting.